Another scheme for the classification of peoples in terms of the highest grade of implements used by them is current under which they are placed in certain ages on the assumption that man in all regions has passed through an orderly sequence in his development, and that the successive changes are expressed by the nature of the material used in the manufacture of implements. Under this plan of classification we have an age of stone, an age of bronze, and an age of iron. The stone age is commonly divided into two parts: an earlier or _paleolithic_, during which the highest type of implement used is fashioned of stone by chipping; and a later, or _neolithic_, when implements of stone are shaped by grinding and polishing. Following the stone age came one of bronze, when a mixture of copper and tin was used for implements; and later the age of iron, beginning when the art of reducing metals from their ores was discovered. In this scheme a copper age is sometimes included, with doubtful propriety, however, if, as in America, the metal referred to is obtained in its native condition. Under the somewhat indefinite scheme of classification just referred to the North American aborigines, inclusive of the Aztecs, etc., previous to the coming of European civilization were in the stone age of development, although bronze was in use among the Incas of Peru, and to some extent had found its way northward as far as Mexico. Certain of the tribes still used implements of chipped stone, but in the great majority of instances implements of polished stone were the highest type known. Native copper was widely used for axes, knives, ornaments, etc., but iron, except such as occurs in meteors, was unknown. The difficulty met with in selecting any one article or any one material used by primitive peoples as a basis for their classification is illustrated by the facts just cited, as it places the lowest savage of America in the same group as the Aztec and the Maya. Obviously, in the classification of peoples as with the lower animals all characteristics should be included. THE ESKIMOS The extreme northern part of North America is included in the circumpolar lands described in another volume of the series of which the present book forms a part, and the Eskimos as a people will therefore receive but passing attention at this time.